elixir: Locate generate_app.escript via defaulted argument

pkgs/development/interpreters/elixir/1.15.nix

···
   sha256 = "sha256-o5MfA0UG8vpnPCH1EYspzcN62yKZQcz5uVUY47hOL9w=";
   # https://hexdocs.pm/elixir/1.15.0/compatibility-and-deprecations.html#compatibility-between-elixir-and-erlang-otp
   minimumOTPVersion = "24";
+  escriptPath = "lib/elixir/scripts/generate_app.escript";
 }

pkgs/development/interpreters/elixir/generic-builder.nix

···
 , sha256 ? null
 , rev ? "v${version}"
 , src ? fetchFromGitHub { inherit rev sha256; owner = "elixir-lang"; repo = "elixir"; }
+, escriptPath ? "lib/elixir/generate_app.escript"
 } @ args:
 
 let
···
   buildFlags = optional debugInfo "ERL_COMPILER_OPTIONS=debug_info";
 
   preBuild = ''
-    patchShebangs lib/elixir/generate_app.escript || true
+    patchShebangs ${escriptPath} || true
 
     substituteInPlace Makefile \
       --replace "/usr/local" $out